In the current state of the art, a typical fuel cell comprises the elements shown in FIG. 1. A membrane/electrode assembly (MEA), 10, comprising a membrane separator, 11, and catalyst coatings, 12, on either side thereof, and two (2) gas difflusion backing sheets, 20, are sealed by gaskets, 30, between two (2) electronically conductive graphite plates, 40. The plates often serve a multiple role as current collectors conveying electrons to the external load via electrical connections not shown, as mechanical supports for the other fuel cell components, and as gas and water distribution networks via a pattern of flow fields inscribed upon the surfaces thereof, 50. Gas and water inputs and outputs are generally integral with the graphite plate, but are not shown. The graphite plates normally serve as the interface between adjacent cells in a stack. The plates are known variously as current collectors, flow fields, and bipolar (or monopolar) plates. For further information, see, for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. Vol. 12A, pp. 55ff, VCH, New York, 1989.
Because of its multiple role, the bipolar plate has a number of requirements to meet. The plate must have good electrical conductivity, good mechanical or structural properties and high chemical stability in the chemically reactive fuel cell environment. In addition because of its gas distribution role it must be made of a gas impermeable material and be formed with complex gas delivery channels across its surface.
In the current practice of the art, graphite is the material of choice for bipolar plates because of its high electrical conductivity, high strength and immunity to corrosion. However, it is brittle, expensive, and requires expensive machining to produce. The brittleness of graphite necessitates its use in ca. six (6) mm thick slabs which adds both weight and bulk to the fuel cell, thus driving down its power density (kW/1 to kW/kg) in use.
Carbon/graphite filled thermoplastic polymers have long been identified as a promising alternative to graphite in bipolar plates. In principle, conductive, reinforced thermoplastic polymer compositions can be molded directly into complex, intricate shaped components using low cost, high-speed molding processes. In addition, these more ductile materials will enable the development of new stack designs because moldable plastics offer much greater flexibility to the form of fuel cell components. Unfortunately, this potential has not been realized in the art despite numerous attempts to do so.
Electrically conductive thermoplastic polymer compositions providing volume resistivities of 10−3−10−2 ohm-cm are known in the art, and are of particular interest in the production of fuel cell current collectors.
U.S. Pat. No. 3,945,844 to Nickols discloses polymer/metal composites. Polysulfone, polyphenylene sulfide, polyphenylene oxide, acrylonitrilebutadiene-styrene copolymer are combined in a variety of ways with stainless steel, silver, gold, and nickel. The amount of either metal powders or fillers or both, in the polymer/metal composite varies from 50 to 80 weight % percent. Resistivity levels as low as 10−3 ohm-cm are reported.
U.S. Pat. No. 4,098,967 to Biddick et al. provides a bipolar plate formed of thermoplastic resin filled with 40-80% by volume finely divided vitreous carbon. Plastics employed in the compositions include polyvinylidene fluoride and polyphenylene oxide. The plates are formed by compression molding dryblended compositions and possess specific resistance on the order of 0.002 ohm-cm. Compression molded bipolar plates from solution blends of graphite powder and polyvinylidene fluoride are disclosed in U.S. Pat. No. 3,801,374 to Dews et al. The plate so formed has a density of 2.0 g/cc and volume resistivity of 4×10−3 ohm-cm.
U.S. Pat. No. 4,214,969 to Lawrance discloses a bipolar plate fabricated by pressure molding a dry mixture carbon or graphite particles and a fluoropolymer resin. The carbon or graphite are present in a weight ratio to the polymer of between 2.5:1 and 16:1. For polymer concentrations in the range of 6-28% by weight, volume resistivity is in the range of 1.2-3.5×10−3 ohm-in.
In U.S. Pat. No. 4,339,322 to Balko et al., the physical strength of the compression molded composite of U.S. Pat. No. 4,214,969 was improved by substituting carbon fibers or other fibrous carbon structures for some of the graphite powder. Typical composition includes 20% (by weight) polyvinylidene fluoride (PVDF), 16% (by weight) carbon fiber, and graphite powder. The dry mixture was blended, then pressure molded into plates. The volume resistivity is in the range of 1.9×10−3 to 3.9 10−3 ohm-in at a binder/resin loading levels of 7-26 wt %.
U.S. Pat. No. 4,554,063-85 to Braun et al. discloses a process for fabricating cathode current collectors. The current collector consists of graphite (synthetic) powder of high purity, having particle sizes in the range from 10 (micron) to 200 (micron) and carbon fibers which are irregularly distributed therein and have lengths from 1 mm to 30 mm, the graphite powder/carbon fiber mass ratio being in the range from 10:1 to 30:1. The binder/resin used is polyvinylidene fluoride. For producing the current collector, the binder is dissolved in, for example, dimethylformamide. Graphite powder and carbon fibers are then added and the resulting lubricating grease-like mass is brought to the desired thickness by spreading on a glass plate and is dried for about 1 hour at about 50° C. The plates were also formed by casting, spreading, and extrusion.
U.S. Pat. No. 5,582,622 to Lafollette discloses bipolar plates comprising a composite of long carbon fibers, a filler of carbon particles and a fluoroelastomer.
Also known in the art is the use of metal-coated, particularly nickel-coated chopped graphite fibers to form conductive polymer compositions. In order to reduce fiber attrition by compounding, the prior art discloses employing a thermoplastic resin-impregnated bundle of nickel-coated graphite fibers which are directly injection moldable with a thermoplastic matrix resin with only a preliminary dry-blending step. See for example Kiesche, “Conductive Composites Find Their Niche,” Plastics Technology, November 1985, P. 77ff; Murthy et al, “Metal Coated Graphite Fiber Structural Foam Composites,” Fourteenth Annual Structural Foam Conference and Parts Competition, The Society of the Plastics Industry, Inc., April 1986, PP 86ff. Use of wider gates and flow channels in molding machines processing graphite fibers is disclosed for example in International Encyclopedia of Composites, S. Lee, ed. pp 474ff, VCH publishers, 1990. Also disclosed therein is the enhancement of conductivity realized by orientation of high aspect ratio conductive fibers in the polymer matrix during the molding process.
Methods for forming resin-impregnated graphite fibers which are also applicable to metal-coated graphite are known in the art. Some of these methods are disclosed in “Graphite Fiber Composites (Electrochemical Processing)” by J. Iroh in Polymeric Materials Encyclopedia, J. C. Salamone, ed., pp. 2861ff, CRC Press 1996.
The art hereinabove cited is directed to replacing pure metal or graphite components which require extensive machining to be formed into finished articles with moldable compositions based upon thermoplastic polymer resins which require less, post-molding machining to form the finished article.
The problem in realizing the advantages of molded thermoplastic polymer parts has been related to the inverse relationship between concentration of conductive filler on the one hand and processability and mechanical properties on the other. In practice, as shown in the art hereinabove cited, quantities of conductive filler required to achieve the 10−2 ohm-cm resistivity goal in fuel cells result in products with limited practical utility. This is particularly true in regard to the formation of current collectors in fuel cell applications.
It is desirable to achieve a combination of properties and processibility in an injection moldable composition without the limitation on practical utility. Another advantage desired is the reduction in cost of forming finished articles such as current collectors in comparison to conventional methods.